The present invention relates to a multi-layer stretch film which can be heat-sealed to polystyrene material.
Stretch films are by definition thermoplastic films that, when applied under tension around a product, elongate and conform to the product.
Stretch films are actually used in the packaging of a substantial number of products including foodstuffs, such as cheese, processed meat, poultry, fruit, vegetable, fish, pizza. etc. These products are currently sold in packages consisting of a base, such as a flat support or preferably a tray, on which the product to be packaged is placed, overwrapped with a stretch film, e.g. stretch PVC or stretch polyolefin films.
Stretch overwrapping is generally carried out using either a horizontal stretch wrapper or an elevator-type stretch wrapper.
In the horizontal stretch wrapper the film is pre-stretched and applied over the product while kept under tension by a suitable grip system. The film is then folded longitudinally around the base supporting the product and sealed longitudinally below said base by means of a centre-sealer. The film tubing is then longitudinally severed and the front and rear flaps thus obtained are folded and welded against the tubing surface by passing the package on a heated belt.
In the elevator-type stretch wrapper, the film is kept tensioned and stretched by raising the product placed on the suitable base against the film. Then the film is folded, both transversely and longitudinally, around the base supporting the product and bunch-sealed against the lower surface of said base by passing the package on a heated belt.
Depending on the type of film employed, passing on a heated belt may be insufficient to close the package by tack welding. In such a case, a pressure-assisted welding step is necessary, wherein a driven overhead pressure roller operates in conjunction with the heated belt.
In any case the tightness of the package closure will vary depending on the welding sealing and cold-/hot-tack) properties of the film.
Depending on the type of material used for the base and for the stretch film or the sealing layer thereof, the welding step may lead to a more-or-less strong bond between the stretch film flaps and between the stretch film and the base. A high bond between the film flaps and a high sticking of the film to the tray would clearly be preferred as the risk of leaking of liquids (i.e. purge) from the package to contaminate the outside of the same package and/or of the other packages that are stored close to it, would thus be reduced.
A new method of packaging using a stretch film has recently been described in co-pending International Patent Application PCT/EP97/3640 filed by the same applicant, the content of which is incorporated herein by reference.
Briefly, said new method provides placing the product to be packaged on the upper surface of a support, such as a flat sheet or a tray, stretching a thermoplastic stretch film over the product, and welding the thermoplastic stretched film to the support all around the product or to the tray flanges so as to form with said support a hermetically sealed enclosure for the product. By this new method the amount of packaging material required is highly reduced, the end package is hermetically sealed with no problems of purge leaks and/or contamination, and the appearance of the package is much more appealing. While in said new method welding can be obtained by any means, welding by heat-sealing is preferred.
Therefore when bases of polystyrene (PS) or foamed or expanded polystyrene (EPS) are employed, what is very common in these food packaging applications, a stretch film that can be heat-sealed to polystyrene is required. While there are many commercially available stretch films with good elastic, optical and mechanical properties, none of these films can be heat-sealed to a polystyrene based surface. On the other hand films that might be heat-sealed to polystyrene-based materials, such as polystyrene films, do not have sufficiently good elastic properties for use as stretch films in these types of packaging applications.
It would therefore be highly desirable to have a stretch film provided with good mechanical, elastic, and optical properties that could also seal to a polystyrene or foamed polystyrene base. Such a film could suitably be employed both in the conventional overwrapping packaging methods and in the new method described in co-pending International Patent Application PCT/EP97/3640 to provide improved packages.
There are a number of properties a stretch film needs to have:
good elastic properties, i.e. good elongation, high elastic recovery and low permanent deformation;
good optics, i.e. low haze and high gloss;
good mechanical properties, in particular good puncture resistance and high abuse resistance;
good sealability to the support material, particularly when used in accordance with the packaging method described in co-pending International Patent Application PCT/EP97/3640;
and, when used in conventional stretch overwrapping, also high cold- and hot-tack properties.
A first object of the present invention is therefore to provide a stretch film with good elastic, optical and mechanical properties that is heat-sealable to polystyrene or polystyrene-based materials.
A second object of the present invention is a stretch overwrapped package wherein the overwrapping stretch film is a film of the invention.
A third object of the present invention is a hermetically sealed package comprising a base of polystyrene-based material, a product to be packaged supported on the upper surface of said base, and a stretch thermoplastic film according to the present invention extending over the product and welded, all around the product, to the base so as to form therewith a hermetically sealed enclosure for the product.